The body - NOT a perfect machine

I have been running steadily all this year, and have logged over 930 miles so far. My runs have been increasing in length in recent weeks. 3 days ago (Saturday morning), I ran 8 miles at Bally - and felt good doing so. But I evidently overdid; the next day I had to stop after 2 miles, as the back of my left thigh was tightening up rather severely. Yesterday I did not try to run at all, as I was experiencing discomfort, and I'm going to take today off, too (giving me consecutive days off for the first time in my memory since resuming running last April). I hope to be back to it tomorrow, as I do feel some improved ... but, we'll see.